Cryptology ePrint Archive: Report 2009/582

A Game-Based Definition of Coercion-Resistance and its Applications

Ralf Kuesters and Tomasz Truderung and Andreas Vogt

Abstract: Coercion-resistance is one of the most important and
intricate security requirements for voting protocols.
Several definitions of coercion-resistance have been
proposed in the literature, both in cryptographic
settings and more abstract, symbolic models. However,
unlike symbolic approaches, only very few voting
protocols have been rigorously analyzed within the
cryptographic setting. A major obstacle is that existing
cryptographic definitions of coercion-resistance tend to
be complex and limited in scope: They are often tailored
to specific classes of protocols or are too demanding.

In this paper, we therefore present a simple and
intuitive cryptographic definition of
coercion-resistance, in the style of game-based
definitions. This definition allows to precisely measure
the level of coercion-resistance a protocol provides. As
the main technical contribution of this paper, we apply
our definition to two voting systems, namely, the Bingo
voting system and ThreeBallot. The results we obtain are
out of the scope of existing approaches. We show that the
Bingo voting system provides the same level of
coercion-resistance as an ideal voting system. We also
precisely measure the degradation of coercion-resistance
of ThreeBallot in case the so-called short ballot
assumption is not met and show that the level of
coercion-resistance ThreeBallot provides is significantly
lower than that of an ideal system, even in case of short
ballots.